A cementitious mixture refers to pastes, mortars, and concrete compositions comprising a hydraulic cement binder. Pastes are defined as mixtures composed of a hydraulic cement binder, either alone or in combination with pozzolans such as fly ash, silica fume, blast furnace slag or calcined clay, and water. Mortars are defined as pastes that additionally include fine aggregate. Concretes additionally include coarse aggregate. These compositions may additionally include other admixtures such as set retarders, set accelerators, defoaming agents, air-entraining or air detraining agents, corrosion inhibitors, water reducing agents, pigments, and any other admixture or additive that does not adversely affect the advantageous results obtained by the present invention.
Dispersants are substances that improve the flow characteristics of the cement slurry by breaking up cement agglomerates and freeing the water, thus giving slurries of lower viscosity and allowing desirable flow conditions to be obtained at lower pump pressures. V. S. Ramachandran, Concrete Admixtures Handbook: Properties, Science, and Technology, Noyes Publications (Second Edition, 1995).
Dispersants have been used in the construction industry to disperse cementitious mixtures. Dispersants such as sulfonated melamine formaldehyde condensate (SMF), sulfonated naphthalene formaldehyde condensate (BNS), and lignosulfonates are commonly used as dispersants. However, these compounds require more than the desired amount of material to achieve a desired level of concrete workability or water reduction. In addition, these materials do not achieve full range (Type A to Type F) water reducing capability, as defined in ASTM C494. For example, lignosulfonates achieve only a low to mid range (5-12%) water reduction before severe set retardation occurs.
Dispersants are a necessary component in high strength and high durability concretes. Due to the requirement for the use of low water amounts in high performance concretes, sometimes high dispersant amounts are necessary to achieve workable concretes. High BNS levels can lead to undesirable retardation of set and may not provide the required workability retention over time.
It is desirable to provide a material that is several times more efficient as a cement or concrete dispersant than the traditional materials like lignosulfonates, BNS and SMF. Improving efficiency reduces the amount of material required to achieve a desired level of concrete workability or water reduction. With respect to the presently used dispersants, lignosulfonates, BNS and SMF, it is also desirable to improve slump retention while maintaining normal setting characteristics. Providing a dispersant with full range (Type A to F) water reducing capability is also a desirable characteristic.
One improvement in the prior art was to use polycarboxylate dispersants. Polycarboxylate dispersants are structured with a polymeric backbone, such as a carbon chain backbone, with pendant moieties. The pendant moieties provide the dispersing capabilities of the dispersant. Polycarboxylate dispersants are mostly polymers with a carbon backbone with pendant side chains, wherein at least a portion of the side chains are attached to the backbone through a carboxyl group or an ether group. For example, polyacrylic acid has carboxylic groups attached to the backbone. Also, side chain moieties such as polyoxyalkylenes can be attached to the carboxylic groups to provide further dispersing capabilities. These dispersants operate by surrounding a particle to be dispersed, and then repulsion forces between each polymer chain keeps the particles apart and more fluid.